This invention relates to a semiconductor junction device, and more particularly to a photovoltaic heterojunction device which is particularly useful as a solar cell.
Conventional solar cells typically comprise a p-n junction formed in a monocrystalline silicon substrate. Typically, an n-type surface region is diffused into a p-type silicon substrate and ohmic contacts are applied. When the device is exposed to solar radiation, photons incident upon the n-type surface travel to the junction and the p-type region where they are absorbed in the production of electron whole pairs. Holes created or which diffuse to the junction region are swept by the built-in voltage to the n-type surface region of the device where they either leave the device as photocurrent or accumulate to produce photo-induced open circuit voltage.
The conversion efficiency of conventional solar cells, however, is seriously limited by a number of factors. First, the built-in voltage is limited by the relatively narrow bandgap of the n-type silicon and the limited extent to which both layers of silicon can be doped. While the built-in voltage of the device can be increased by increased doping of both layers forming the junction, such excess doping tends to reduce conversion efficiency by reducing the lifetime of the carriers and thereby the collection efficiency of the device. As a consequence, the open circuit voltage of a typical silicon solar cell is only about 50% of the silicon bandgap. Secondly, silicon tends to absorb high energy photons, i.e. blue and ultraviolet light, very near the surface and typically within a micron thereof. As a consequence, many of the high energy photons are absorbed near the surface of the n-type region, causing carriers generated by such absorption to recombine at the surface and become lost as sources of photocurrent. Still a third limiting factor resides in the fact that lower energy photons, i.e. red and near infrared light, tend to penetrate deeply into the silicon before they are absorbed. While minority carriers created by deep absorption can contribute to the photocurrent if minority carrier lifetimes are sufficient to permit them to drift into the junction region, the high temperature diffusion step required to form the n-type region significantly reduces minority carrier lifetime in the p-type silicon substrate. As a consequence, many carriers created by deep absorption are lost as sources of photocurrent.
Accordingly, it would be highly desirable to overcome some of the aforesaid problems associated with conventional type solar cells, which would improve performance by providing improved conversion efficiency.